The present invention relates to a method for continuous casting of metals, in which a liquid metal is supplied as a jet to a mould, which already contains further liquid metal. The invention also relates to a device for continuous casting of metals, comprising a mould, through which a liquid metal passes, and a member through which a liquid metal is supplied as a jet to the mould, in which there is already liquid metal.
In continuous casting a liquid metal is supplied to a mould, in which it is cooled and shaped to an elongated string. Depending upon the cross section dimensions the string is called xe2x80x9cbilletxe2x80x9d, xe2x80x9cbloomxe2x80x9d or xe2x80x9cslabxe2x80x9d. A primary flow of hot, liquid metal is during the casting delivered to a cooled mould, in which a metal is cooled and at least partially solidifies into an elongated string. The cooled and partially solidified string leaves the mould continuously. At the point where the string leaves the mould it has at least a mechanically self-supporting solidified skin surrounding a centre part not solidified. The cooled mould is open at two opposite ends as seen in the casting direction and preferably connected to means for supporting the mould and means for supplying cooling means to the mould and the support means. The mould is preferably made of an alloy with a copper base and a high thermal conductivity.
The liquid metal is supplied to the mould from a casting box through a tube extending down into the mould. The tube extends preferably that far into the mould that it projects into the liquid metal preferably present there. When the liquid metal from the tube flows into the liquid metal already present in the mould, it generates a so-called primary flow and a so-called secondary flow. The primary flow goes downwardly in the casting direction, while the secondary flow goes from the region of the walls of the mould upwardly towards the surface of the metal bath located therein and downwardly. In different parts of the metal bath present in the mould periodic velocity oscillations are created during the casting sequence. Thus, upper and lower loops in which the liquid metal flows around are formed in a way known per se. As a consequence of resonance phenomena, which are associated with periodical oscillations of such loops, large bubbles, for instance argon bubbles, oxide enclosures from the casting tube and slag from the meniscus will be transported far downwardly in the casting direction, i.e. far downwardly in the casting string initially formed in the mould. This results in enclosures and irregularities in the final, solidified casting string.
Velocity variations caused by oscillating flow in the mould gives rise to pressure variations at the meniscus, and meniscus height variations. At high meniscus velocities, this results in
(a) draw-down of slag,
(b) uneven slag thickness,
(c) uneven skin thickness, and
(d) a risk of crack formation.
Furthermore, the oscillating flow results in an asymmetric velocity downwardly in the mould. The velocity may in some positions at one narrow side get substantially higher than at the other. This gives rise to a strong transport downwardly of enclosures and gas bubbles accompanied by a decreased slag quality.
The prior art relates to different devices and methods for influencing the primary and the secondary flows, respectively, in the liquid metal in the mould. The prior art utilizes for this devices for applying substantially static magnetic fields during the casting sequence over at least a part of the liquid metal contained in the mould. It is for instance known through the Swedish patent publication SE 436 251 to arrange a static direct current magnetic field or permanent magnetic field at the mould. It may as an a alternative be formed by a low frequency alternating current field having a frequency below 1 Hz. When the metal flowing in passes this field the movement of the tap jet into the rest of the liquid metal is retarded, wherethrough the flow picture is influenced favourably for the casting sequence. This technique has then been further developed. The magnets used for the generation of the magnetic field have for example been placed so that a magnetic field at different levels of the mould in the casting direction has been obtained, whereby specific local movements in the liquid metal could be influenced separately through the respective magnetic field. It has also been proposed to arrange the magnets and the yokes connecting them in such a way that the magnetic fields extend in the casting direction in stead of transversely thereto.
An object of the present invention is to provide a method substantially disturbing and thereby reducing the generation of periodical oscillations and resonance phenomena associated therewith, which are commonly existing in a liquid metal in a mould in connection with continuous casting of metals.
This object is obtained by means of a method of the type defined in the introduction, which is characterized in that a magnetic field varying with time and being substantially fixed in the room is applied by means of magnet members to the liquid metal in the mould for preventing natural oscillations of the liquid metal from being generated. The field will function as a damper in the liquid metal.
According to a preferred embodiment of the method the application of the magnetic field is carried out periodically. The periodicity may be adapted to the periodicity of the oscillations, which normally for casting steel slabs is in the order of 1-30 seconds. The field is applied periodically for disturbing and eliminating oscillations in the liquid metal without changing the main flow topology therefor. The flow topology may be changed, for example by means of strong, static magnetic field or a moving field.
According to a further preferred embodiment of the method the varying magnetic field is applied at irregular intervals during the casting sequence. Thanks to the irregularity of the application it is avoided to regularly amplify certain regular, periodical oscillations in the liquid metal. The varying magnetic field causes instead thanks to the regularity thereof a disturbance of such regular natural oscillations in the liquid metal.
According to a further preferred embodiment the irregular application of the magnetic field is carried out at random times. The magnetic field applied randomly counteracts and disturbs efficiently the generation of periodical oscillations. The random application of the magnetic field results in a minimum risk of possibly amplifying any natural oscillation present in the liquid metal during any extended time.
According to an alternative embodiment of the method the periodical application of the magnetic field is carried out at predetermined times. These times are preferably times known in advance, at which periodical oscillations in the liquid metal are in a certain critical stage, for instance when resonance phenomena caused by said natural oscillations start or may start to occur. The predetermined times are then based on practical observations or calculations of the time for the occurrence of such critical stages in the liquid metal under given casting conditions or measurements of meniscus deformation.
According to a further preferred embodiment of the method the application is carried out upon detection of a certain state in the liquid metal. Said state is preferably a predetermined detectable movement in the liquid metal or as an alternative of the meniscus.
According to a further preferred embodiment the varying magnetic field is provided with a stochastically varying amplitude. The probability for disturbing and not amplifying the natural oscillations generated in the liquid metal during the casting sequence is thereby increased.
According to a further preferred embodiment the varying magnetic field is given a frequency being in the order of 10-103 times higher than the frequency of the oscillation or oscillations in the melt intended to be disturbed thereby. These are normally of the type wide-band spectrum. A very reliable disturbance of said oscillations of the liquid metal is thereby obtained. The magnetic field may then advantageously be applied only during a restricted part of the period of the oscillation or oscillations and nevertheless give a satisfying and reliable disturbance influence.
A further object of the invention is to provide a device, by means of which natural oscillations and resonance phenomena associated therewith of a liquid metal in the mould during continuous casting of metals may be disturbed and prevented from being generated.
This object is obtained by means of a device of the type defined in the introduction, which is characterized in that it comprises magnet members for applying a magnetic field varying with time and being substantially fixed in the room to the liquid metal in the mould. Thanks to the fact that the magnetic field varies it may easily be controlled, i.e. be given such an amplitude and frequency, that the periodical oscillations existing or generated in the liquid metal are efficiently disturbed.
According to a preferred embodiment the magnet members are arranged in such a way that they generate the varying magnetic field periodically during the casting sequence. The periodicity is advantageously adapted to the periodicity of the oscillations, which is in the order of 1-30 seconds when casting steel slabs.
According to a further preferred embodiment the magnetic members are adapted to generate the varying magnetic field at irregular intervals. This disturbs efficiently a generation of each natural oscillation, since they will with a high probability be in off-phase with the periodical oscillations existing in the liquid metal.
According to a further preferred embodiment the magnet members are adapted to generate a magnetic field having a stochastically varying amplitude. They are preferably also adapted to generate a magnetic field having a varying frequency, for example stochastically varying frequency, within a given frequency interval. Thanks to the variation of the amplitude and/or frequency of the magnetic field, or the current used to generate the magnetic field, a very reliable disturbance of the natural oscillations in the liquid metal is obtained, at the same time as an amplification of possibly natural oscillations in the liquid metal are efficiently avoided.
According to a further preferred embodiment the magnet members are adapted to generate a substantially static magnetic field, onto which the varying magnetic field is superposed. The static magnetic field is preferably used for influencing the so-called primary flows and secondary flows in the liquid metal in the mould, which the liquid metal supplied thereto gives rise to. The same type of basic equipment, which has according to the prior art been used for achieving such an influence, may accordingly advantageously be used for the combined function aimed at, since both a static magnetic field and varying magnetic field superposed thereon are applied to the liquid metal in the mould.
According to a further preferred embodiment of the device the varying magnetic field has a frequency in the order of 10-103 times higher than the frequency of the oscillation or the oscillations of the liquid metal to be disturbed thereby. A very reliable disturbance of the natural oscillations in the liquid metal is thereby obtained.
Further characteristics and advantages of the invention appear from the other dependent claims and the following detailed description.